A variety of image-display/image-projection devices and techniques are available for visually displaying/projecting graphical or video images—often called video frames—to a viewer. Typically, a graphical image is an image that changes slowly or not at all. For example, a flight-instrument graphic is an image of cockpit instruments that overlays a pilot's view. This graphic may be projected onto a viewing area such as the windshield or may be projected directly into the pilot's eyes such that he/she sees the flight instruments regardless of his/her viewing direction. There is typically little change in this graphic other than the movement of the instrument pointers or numbers. Conversely, video frames are a series of images that typically change frequently to show movement of an object. For example, a television set displays video frames.
A cathode-ray-tube (CRT) display, such as used in a television or a computer monitor, is a common image-display/image-projection device that, unfortunately, has several limitations. For example, a CRT is typically bulky and consumes a significant amount of power, thus making it undesirable for many portable or head-mounted applications.
Flat-panel displays, such as liquid-crystal displays (LCDs), organic LEDs, plasma displays, and field-emission displays (FEDs), are typically less bulky and consume significantly less power than a CRT having a comparable viewing area. But, flat panel displays often lack sufficient luminance and adequate color purity and/or resolution for many head-mounted applications.
A common problem with both CRTs and flat-panel displays is that the displayed/projected image may include visible artifacts that are introduced into the image during the capturing, processing, or displaying of the image. Typically, an image-capture device such as a vidicon tube or charge-coupled device (CCD) captures an image of an object by converting light reflected by the object into electrical signals. A display/projection system that includes one of the aforementioned display/projection devices receives these electrical signals and processes them. The display/projection device converts these processed electrical signals into an array of pixels, which a viewer perceives as an image of the object. Unfortunately, visible errors and degradations, often called artifacts, may be introduced into the image during the conversion of the reflected light into electrical signals, during the processing of the electrical signals, or during the converting of the electrical signals into pixels.
Recently, engineers have developed an image amplifier that can display an image or project the image onto a display screen. Typically, an image amplifier is less complex, less expensive, and can be made smaller than a CRT or flat-panel display, and an image-amplifier display system typically uses significantly less power than a CRT or flat-panel display system. Furthermore, because it does not necessarily convert light into electrical signals and back again, an image-amplifier system typically introduces fewer artifacts into an image.
FIG. 1 is a perspective view of a conventional image-amplifier display system 20 that includes an image amplifier 22, an illuminator 24, and an image generator 26. For example, the image amplifier 22 may be a Light Smith, which was developed by Simac Company of Boise, Id. Although, as discussed above, the system 20 is often less complex, cheaper, and smaller than a CRT or flat-panel display system, it can display/project a relatively bright and high-quality image 28.
The image amplifier 22 of the system 20 includes transparent front and back electrodes 30 and 32 and a display/projection screen 34 having a display/projection surface 36 and a scan surface 38. An electric-field generator (not shown) is coupled to the electrodes 30 and 32 and generates an electric field across the screen 34. This electric field allows the image generator 26 to set the brightness levels—here the reflectivity levels—of the regions of the display/projection surface 36 such that the generator 26 can generate bright and dark pixels of an image. For example, the generator 26 can set the reflectivity of the region 44 on the surface 36 to a relatively high level such that the region 44 reflects a relatively high percentage of the incident light from the illuminator 24. Therefore, in this example, the pixel of the image 28 corresponding to the region 44 is a relatively bright pixel.
The illuminator 24 typically includes an incoherent light source such as an incandescent bulb (not shown), which illuminates the display/projection surface 36 of the screen 34. The surface 36 reflects the light from the illuminator 24 according to the reflectivity of each region 44 to display the image 28—that is, project the image 28 directly into a viewer's (not shown) eye—or to project the image 28 onto a display screen 46 through an optical train 47, which is represented by a lens.
The image generator 26 generates the image 28 on the display/projection surface 36 of the screen 34 by erasing the surface 36 with an electromagnetic erase burst 40 and then scanning an image beam 42 across the scan surface 38.
More specifically, erasing the surface 36 of the screen 34 entails simultaneously setting all the regions 44 on the surface 36 to the same or approximately the same predetermined reflectivity level with the erase burst 40. Typically, this predetermined reflectivity level is a low reflectivity level—which represents black—although it can be any other desired reflectivity level. The erase burst 40 typically is an energy burst having a first wavelength in the visible, ultraviolet, or infrared range of the electromagnetic spectrum and is wide enough to simultaneously strike the entire scan surface 38. Where the erase level is black, the screen 34 is typically constructed such that exposing the scan surface 38 to this first wavelength reduces the reflectivity levels of the regions 44. Because these reflectivity levels may be different from one another before the erase cycle, the generator 26 generates the burst 40 long enough to reduce the reflectivity levels of all the regions 44 to the black level regardless of their pre-erase reflectivity levels. Furthermore, because it typically “turns off” the reflectivities of the regions 44, the burst 40 is sometimes called an “off” burst.
Generating the image 28 on the screen 34 entails scanning the image beam 42 across the scan surface 38 to set the reflectivity levels of the regions 44 such that the reflectivity levels correspond to the brightness levels of the respective pixels of the image 28. The beam 42 typically is an energy beam having a second wavelength in the visible, ultraviolet, or infrared range of the electromagnetic spectrum and has a diameter that equals or approximately equals the diameter of a region 44. Typically, one can set the diameter of the beam 42—and thus the diameter of each region 44—small enough so that the image amplifier 22 provides a high-resolution, high-quality image 28. Where the erase level is black, the screen 34 is typically constructed such that exposing the scan surface 38 to this second wavelength increases the reflectivity levels of the regions 44. The image generator 26 sets the reflectivity level of a region 44 by modulating the time that the image beam 42 strikes the region of the scan surface 38 corresponding to the region 44, by modulating the intensity of the beam 42 as it strikes the corresponding region of the surface 38, or by modulating both the intensity and time. The generator 26 can modulate the intensity of the beam 42 by modulating the power to the beam source (not shown) or with an acoustic-optic modulating crystal (not shown) in the path of the beam 42. Because the reflectivity level of a region 44 starts out at a known level—black for example—the generator 26 can use a look-up table or other techniques to determine a striking time or striking intensity that will set the region 44 to the desired reflectivity level. Furthermore, because it effectively “turns on” the reflectivities of the regions 44, the beam 42 is sometimes called an “on” beam.
Still referring to FIG. 1, in operation for still images, the image generator 26 generates the erase burst 40 to erase the surface 36 of the screen 34, and then scans the image 28 onto the surface 36. Specifically, the generator 26 scans the image beam 42 across the scan surface 38 of the display screen 34 to generate the image 28 on the surface 36. In one embodiment, the persistence of the surface 36 is relatively long such that once the beam 42 scans the image 28, the screen 34 “holds” the image. “Persistence,” as used in reference to FIG. 1, is the amount of time that a region 44 of the surface 36 retains the reflectivity level set by the beam 42. Therefore, if the persistence is relatively long, e.g., hours, then the generator 26 need not rescan the image 28 or may rescan the image 28 at relatively long intervals. The generator 26 may scan the beam 42 according to any number of conventional scanning techniques such as those described in U.S. Pat. No. 6,140,979, entitled “Scanned Display With Pinch, Timing, And Distortion Correction”, which is incorporated by reference.
In operation, for a series of video frames, the image generator 26 generates the erase burst 40 before each frame, and then scans the image beam 42 across the surface 38 to generate the frame on the display/projection surface 36. The generator 26 then repeats this sequence—generating an erase burst 40 and then scanning the surface 38 with the beam 42—for each frame.
Unfortunately, a problem with generating video frames on a long-persistence screen 34 is uneven brightness control within frames. Specifically, if the entire region is erased, the first region 44 scanned by the image beam 42 has the desired reflectivity level for a longer time than the subsequently scanned regions, and for a significantly longer time than the last-scanned region. Therefore, the first-scanned regions 44 may appear brighter on average than the last-scanned regions. For example, assume that the image generator 26 generates the erase burst 40 every T seconds, scans a first region 44 with the image beam 42 virtually immediately after generating the erase burst 40, and generates the next erase burst 40 t seconds after scanning the last region 44. Therefore, the first-scanned region 44 has its “on” reflectivity for approximately T seconds, while the last-scanned region 44 has its “on” reflectivity for only t seconds. Consequently, because the first-scanned regions 44 tend to be “on” longer than the last-scanned regions 44, the first-scanned regions tend on average to appear brighter to the eye than the last-scanned regions 44. Thus, this may cause the image 28 to have an uneven brightness.
One approach to addressing uneven brightness is to shorten the persistence of the screen 34. For instance, referring to the above example, one can construct the screen 34 such that the regions 44 each have a persistence of approximately t seconds such that each region 44, regardless of when it is scanned by the image beam 42, has its “on” reflectivity for approximately the same time.
While this solution may reduce the appearance of uneven brightness, it will often reduce the overall brightness of the image 28, or may otherwise degrade the image 28.